Environmental Engineering Reference
In-Depth Information
are often due to loss of rare alleles, which typically have
little effect on overall heterozygosity. It is clear that
reintroduction programmes should attempt to create
populations with high levels of genetic diversity. Start-
ing with the highest possible number and ensuring a
high initial population growth rate will help to main-
tain high genetic diversity, but the effects of genetic
diversity on the success rate of reintroductions is still
to be further explored (see also Chapter 7).
Sometimes, insights in
metapopulation
theory
may be used to understand the success rate of re-
introductions. Singer
et al
. (2000) related the fate of a
number of bighorn sheep (
Ovis canadensis
) reintroduc-
tions to this species naturally occurring in metapopu-
lations. At present, most extant populations of bighorn
sheep consist of fewer than 100 individuals occurring
in a
fragmented
distribution pattern whereas the
species formerly occupied a more continuous and
wider range. Singer
et al
. (2000) investigated the cor-
relates for the rate of colonization of 79 suitable, but
unoccupied, patches by 31 translocated populations of
bighorn sheep released into nearby patches of habitat.
Dispersal rates were 100% higher in rams than in
ewes. Successful colonizations of unoccupied patches
(24 out of 79 patches were colonized) were associated
with rapid growth rates of the released population
(Figure 8.3), years since release (Figure 8.4), larger
area of suitable habitat in the release patch, larger
population sizes and a seasonal migration tendency in
the released population (Figure 8.5). In this study area,
colonization rates were much higher than other studies
have reported (Singer
et al
. 2000), and this could be
attributed to the presence of larger regions of unoc-
cupied suitable habitat with a greater probability for
10
8
6
4
2
0
0-5
6-10
11-15 16-20
20+
Years since release into first patch
Figure 8.4
Probability of successful colonizations in
relation to the number of years since release in the fi rst
patch for 31 translocated populations of bighorn sheep
released 1947 - 1991. (Modifi ed from Singer
et al
. 2000 .
Reproduced by permission of Wiley-Blackwell Publishers.)
60
50
100
40
80
30
60
20
40
10
0
20
Non-
migratory
Partially
migratory
Migratory
0
Migratory tendency
-1.05 1.06-
1. 1 0
1.31+
Growth rate (
l
) of translocated population
1. 11 -
1. 1 5
1. 1 6 -
1.20
1.21-
1.25
1.26-
1.30
Figure 8.5
Probability of successful colonizations in
relation to migratory tendency in the release patch for 31
translocated populations of bighorn sheep. Migratory, > 75%
of the population uses distinct seasonal changes; partially
migratory, part of the population migrates; nonmigratory,
year-round use of the same ranges. (Modifi ed from Singer
et al
. 2000. Reproduced by permission of Wiley-Blackwell
Publishers.)
Figure 8.3
Probability of successful colonizations of new
patches is correlated with population growth rates (λ) of 31
translocated populations of bighorn sheep in the western
United States in 1946-1997. (Modifi ed from Singer
et al
.
2000. Reproduced by permission of Wiley-Blackwell
Publishers.)
